Steam turbine rotor cooling arrangement



Feb. 25, 1969 R. E. BRANDON ET AL 3,429,557

STEAM TURBINE ROTOR COOLING ARRANGEMENT Filed June 50, 1966 v"INVENTORS'.

RONALD E. BRANDON,

RUSSELL J. HOLMAN, BY m flw THEIR ATTORNEY.

United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE A steam turbinein which the rotor parts which are subjected to the hottest steam arecooled by relatively cool steam. A flow path arrangement to recirculatepartially expanded steam from a lower stage is provided for thispurpose.

This invention relates to elastic fluid turbines generally and, moreparticularly, it relates to a cooling arrangement for the rotors ofsteam turbines.

In the turbine art, it is well known that higher operating temperaturescan result in more efficient thermodynamic cycles. In the present stateof the art, initial steam temperatures of 1000 F. are commonly used.While thermodynamic considerations tend to increase operatingtemperatures, other considerations such as cost of special material andoperating difiiculties with steels of high alloy content make suchtemperatures diflicult to justify economically. It is thus common to usespecial materials only in critical areas such as rotor buckets andnozzle partitions, while employing standard materials for such parts asrotor bodies. In order to permit the use of a standard material for aturbine rotor body, it is necessary to provide some arrangement forkeeping it at a temperature below the elevated temperatures to which thebuckets are subjected. This problem presents itself in the initial stageof any group of turbine stages, where the steam is at its highesttemperature.

By way of example, and not of limitation, one environment for thepresent invention is in a double flow, low pressure steam turbine.Because of rotational stress and long term metallurgical properties ofthe rotor, rotor temperature at the hottest portion must be limited toapproximately 650 F. This hottest portion of the rotor for a double fiowlow pressure turbine would normally be in the region of the annularwheel space between its first stage wheels. A combination of leakage,convection, and radiation from the much hotter steam in the low pressureinlet, plus heat generated by fluid friction between rotating andstationary parts, would normally result in temperatures much higher thanthe 650 F. limit. This particular temperature limitation has notheretofore been a serious problem because steam temperatures approachingthe low pressure turbine were generally not higher than about 650 F.Steam temperatures have increased, however, so that it is now essentialto provide some method and means of cooling the rotor exposed to thistype of duty.

Accordingly, it is an object of the present invention to provide animproved arrangement for cooling the body of a turbine rotor where it isadjacent to the hottest motive fluid.

Another object is to provide a reliable arrangement for turbine rotorcooling which is efiective despite changes, during use, in theclearances of the labyrinth seals between rotor and casing.

Another object is to provide a reliable means to motivate coolant fluidto the critical areas of a turbine rotor.

Other objects, advantages and features of the present invention willbecome apparent from the following de- 3,429,557 Patented Feb. 25, 1969scription of one embodiment thereof, when taken in connection with theaccompaying drawing in which:

FIG. 1 is a partial longitudinal elevation, partly in section, of amultistage, axial flow steam turbine according to the present invention;

FIG. 2 is a detail view taken along the line II-II FIG. 1.

Briefly stated, the present invention is practiced in one form by a lowpressure turbine having holes through its first stage wheel. On thedownstream face of the wheel and projecting into the wheel space arescoops assisting the flow of steam from the discharge of the first stageback through the wheel holes and into the wheel space up stream of thefirst turbine stage. In the turbine casing is a recirculation conduitproviding communication from the flow path downstream of the firstturbine stage to the upstream wheel space. First stage rotor bucketshave negative reaction at their root port-ions causing slightly lowerpressure on the upstream side thereof than on the discharge side. Thelower pressure at the first stage upstream wheel combined with thescooping action through the wheel holes, causes relatively cool steam toflow from the first stage discharge to the wheel space upstream of thefirst stage and then to the upstream bucket root portions. Depending onwhether fiow from the scoops is more or less than allowed by thelabyrinth seals, flow will go through the recirculation conduit eitherout of or into the upstream wheel space, respectively. Thus, the hottestparts of the turbine rotor are cooled.

Referring now to FIG. 1 in the drawing, a turbine rotor is generallyindicated at 1. Axially spaced along rotor 1, are first and subsequentstage rotor wheels 2 and 2a respectively, which carry turbine buckets 3and 3a respectively which are in turn surrounded by shroud bands 6. Theturbine casing, generally indicated at 4, includes nozzle partitions 5,stationary shrouds 7, diaphragm members 8, and labyrinth seals 9.Stationary shrouds 7 cooperate with the sealing strips on shroud bands 6to restrict steam flow therebetween. Likewise, labyrinth seals 9cooperate with rotor body 1 to restrict steam flow therebetween. Seals 9include strips 19 inclined as shown to permit a higher flow coefiicienttherearound in the direction of inclination than in the oppositedirection. The axially spaced rotor wheels 2 and 2a are separated bywheel spaces shown at 10 and 11. Wheel space 11 is in the center of therotor, that is to say, adjacent to the steam inlet or bowl 16 andupstream of the first turbine stages. Wheel spaces 10 and 11 are annularchambers, substantially defined by rotor wheels 2, 2a, diaphragm members8 and other stationary casing members.

First stage rotor Wheel 2 has a plurality of circumfer entially spacedaxial holes 13 therethrough such that wheel spaces 10 and 11 communicatewith each other through the wheel 2. Extending from these holes 13 andinto downstream wheel space 10, are a plurality of scoops 12. On theupstream face of first stage rotor wheel 2 is a root area 15, thuslabeled for convenience. A recirculation conduit 14 communicates at oneof its ends with wheel space 11. At its other end, recirculation conduit14 communicates with the interior of the turbine casing 4 in thevicinity of the tips of first stage rotor buckets 3, downstream thereof.

First stage rotor buckets 3 are designed for negative reaction at theirroots. That is to say, that in the area 15 adjacent their roots, theupstream steam to the first stage buckets 3 is at a slighlty lowerpressure than the discharge steam from first stage buckets 3, becausethe root portion of the bucket is actually functioning as a diffuser tocause the negative reaction.

In opertion, wheel space 11, since it communicates by way ofrecirculation conduit 14 and axial holes 13,

with the steam path downstream of first stage rotor buckets 3, is at asteam pressure approximately equal to the steam pressure downstream offirst stage rotor buckets 3, and possibly at a slightly higher pressuredue to the supercharging effect of scoops 12. In addition, due to thenegative reaction of buckets 3, pressure at root areas 15 is less thanin space 11. Thus, there is flow of relatively cool steam from scoops 12to wheel space 11 and then past labyrinth seals 9 to the lower pressureroot areas 15 on the upstream sides of first stage rotor wheels 2. ThisHow must be of sufiicient magnitude to absorb heat generated by fiuidrotational friction in the wheel spaces 10 and 11 plus other heatconducted to space 11 from the bowl 16. Such heat entering space 11 maybe due to leakage of higher pressure, higher temperature steam throughnumerous small unavoidable leakage paths or spaces. This heat must beabsorbed without permitting the rotor surface temperature to go above650 F. At this point, there are two distinct possibilities in the modeof operation of the present invention.

In the first, assuming the labyrinth seals 9 to be relatively tight, andpermitting relatively small flow from wheel space 11 to first stage rootareas 15, the general flow direction starting at scoops 12 is asfollows: Steam downstream of first stage rotor buckets 3 is scooped inthrough axial holes 13 by scoops 12 and enters wheel space 11. Fromwheel space 11, a small portion flows past labyrinth seals 9 to cool thelower pressure root areas of first stage rotor wheel 2. It is desirableto keep this flow small since large flows at this location aredetrimental to turbine efliciency. Due to the fact that scoops 12provide more steam than can normally flow past labyrinth seals 9, theexcess flowing into wheel space 11 is caused to flow out therefromthrough recirculation conduit 14 and into the turbine casing at a pointdownstream of first stage rotor buckets 3, thus maintaining sufiicientflow rate to keep the wheel space 11 cool. Flow through wheel space 11and conduit 14 is thus motivated by the pumping action of scoops 12.

The second possibility arises where labyrinth seals 9 are relativelyworn permitting greater flow therearound. If insufiicient cooling flowwere available, that is to say, without recirculation conduit 14, thepressure in wheel space 11 would decrease. This would result because offree communication with one or both of lower pressure root areas 15(which as a practical matter are never equal in pressure) to the pointwhere hotter steam would be drawn from one root area 15 into wheel space11 and out to the other root area 15, depending on which area 15 happensto have the lower pressure, thus defeating the cooling system. In thepresent invention, however, starting with steam within the casingdownstream of first stage rotor buckets 3, the flow is as follows:Scoops 12 motivate the steam through axial holes 13 and into wheel space11 from which the flow continues relatively freely past labyrinth seals9 to lower pressure root areas 15 on the upstream side of rotor wheels2. If there is still a capacity for the flow of coolant steam into therelatively low pressure root areas 15, coolant steam flows from thecasing downstream of the tips of the first stage rotor buckets 3 andthrough the recirculation conduit 14, and into wheel space 11 tosupplement the flow thereinto through axial holes 13.

Thus, relatively cool steam discharging from the first stage rotorbuckets 3 is used to cool the turbine rotor in the vicinities of wheelspace 11 and the root areas 15 of first stage rotor wheels 2, regardlessof the condition or performance of labyrinth seals 9. The pressure inwheel space 11 is prevented from decreasing to the point where hot steamfrom areas 15 can be drawn into space 11. In this arrangement, flowthrough conduit 14 is motivated by the differential between first stageexhaust steam pressure and the lower steam pressure at root areas 15,due to negative reaction at the bucket roots.

It will thus be apparent that an effective arrangement 4 I has hereinbeen described for cooling a turbine rotor in the region where it isadjacent to the hottest motive fluid. The present arrangement iseffective despite changes, during use, in seal clearances.

In the foregoing description, of a double flow, low pressure turbine,wheel holes 13 have been provided in the first stage wheels 2 on onlyone side of the turbine. It is of course within the contemplation of theinvention to use them on both sides if required.

Furthermore, it will occur to others of ordinary skill in the art tomake other modifications of the present invention which will lie withinthe concept and scope thereof and will not constitute patentabledeparture therefrom. For example, it is within the contemplation of thisinvention that it be useful on steam turbines of all pressure ranges, aswell as gas turbines, in addition to the above-described double-flow,low pressure turbine environment. Therefore, it is intended that theinvention be not limited by the details in which it has been describedbut that it encompass all within the purview of the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A steam turbine including a casing and a rotor defining a motivesteam path therebetween, said rotor having a row of buckets mounted on afirst-stage wheel and disposed in said path, said buckets being shapedover their major outer portions to extract energy from the motive steamwith a reduction in pressure across the bucket row, said buckets havingroot portions also disposed in said path and shaped for negativereaction so as to extract energy as well as to provide a region on theupstream side of the row in the vicinity of the bucket root which islower in pressure than on the downstream side,

a rotating seal on the upstream side of the first-stage wheel formingclose clearances between the casing and the rotor and defining togethertherewith an upstream wheel space on one side of the seal and providinga restricted flow path from said wheel space across the wheel into saidregion in the motive steam path,

means for motivating the cooler expanded steam from the downstream sideof the bucket row to said wheel space without significant pressure losswhen the wheel is rotating.

2. The combination, according to claim 1, wherein said motivating meansincludes holes through said first-stage wheel and scoops mounted on thewheel and arranged to motivate steam upstream through said holes.

3. A steam turbine including a casing and a rotor defining a motivesteam path therebetween, said rotor including buckets mounted on axiallyspaced rotor wheels,

said casing and said rotor further defining wheel spaces on the upstreamand downstream sides of the first stage rotor wheel,

a rotating seal forming close clearances between said casing and rotoron the upstream side of said first stage rotor wheel, said sealrestricting communication between said flow path, and said upstreamwheel space and being subject to variations in flow there through,

the first stage rotor wheel defining a plurality of holes therethroughand having a scoop mounted thereon on the downstream side thereofadjacent each of said holes,

the first stage rotor buckets shaped to provide a negative reaction attheir root portions when the turbine is operating, so that the motivesteam is at a higher pressure on the downstream side of said bucketroots than on the upstream side thereof,

a stationary recirculation conduit communicating with the downstreamside of said first stage rotor buckets and with said upstream wheelspace,

said scoops and the lower pressure on the upstream side of the firststage bucket root portions motivating 5 6 elastic fluid flow throughsaid holes, said upstream References Cited wheel space, and said sealclearances to the upstream UNITED STATES PATENTS side of said firststage rotor wheel, said recirculation conduit carrying excess elasticfluid 3206166 9/1965 Beldecos et 253' 391 2,552,239 5/1951 Warren 60--4Ofrom said upstream wheel space when the flow 5 3 189 3 1 65 B l 1 25 3915 through the rotor wheel holes is greater than through 20 6/ 9 e decoset a 3 the clearances, and said recirculation conduit carrying elasticfluid into said FOREIGN TE upstream wheel space when said clearances arerela- 6351890 4/1950 Great Bntamtively large and the flow through therotor wheel 10 holes is adequate to maintain pressure in said up-EVERETTE POWELL Prlmw'y stream wheel space.

